The present invention relates to broadband receivers, and more particularly to broadband receiver amplitude/phase normalization using a broadband temperature compensated noise source and a pseudo random sequence generator.
A broadband receiver does not have constant gain across its input frequency range. However a receiver in test equipment such as a spectrum analyzer requires that the gain be constant in order to measure the amplitude of an input signal accurately. Typically to achieve this an amplitude calibration is performed where a known level input signal from a calibrated signal generator is stepped through the input bandwidth and the differences in gain with frequency are recorded and used by the instrument firmware to correct the amplitude response of the receiver.
The broadband receiver also has group delay and amplitude variations over single channel bandwidths mainly due to the narrowband filters required in intermediate frequency (IF) stages of the receiver. In a receiver where the final IF stage output is digitized and processed, the amplitude and frequency response of the IF stages may be equalized by injecting a known pseudo random sequence and correlating them in the digital signal processing.
One problem with these calibration steps is that the overall receiver gain and the IF filter response change with temperature subsequent to the calibration. This invalidates the calibration that was performed.
For a broadband receiver that may be used easily in the field for amplitude/phase normalization whenever necessary, what is desired is a self-contained calibration system which is temperature stable.
Accordingly the present invention provides broadband receiver amplitude/phase normalization using a broadband temperature compensated noise source and a pseudo random sequence (PRS) generator. A broadband receiver has internal temperature compensated normalization sourcesxe2x80x94a noise source and a PRS generator. A receiver relay couples either a signal from an input port or the normalization sources to the receiver. A calibrated source provides a calibration signal at stepped frequency intervals to the receiver with the relay in a first position to generate a table of gain versus frequency. Then the internal noise source is switched in at the same stepped frequency intervals to produce a table of noise level versus frequency. Next the PRS generator is switched in at the lowest input frequency for the receiver to produce filter coefficients for an IF equalization filter. Since the normalization sources are temperature stable, they may be switched in and measured by the receiver at any time to normalize any subsequent changes in the broadband receiver gain or changes in the IF linear distortions due to changing temperature conditions.
The objects, advantages and other novel features of the present invention are apparent from the following detailed description when read in conjunction with the accompanying claims and attached drawing.